Junction-type metal/metal oxide solid state pH electrodes have been proposed for sensing the pH of solutions and other fluids. These electrodes have the sought after advantages of stability in aqueous solutions over a wide range of temperatures and pressures, low impedance and fast response to pH changes. Fog et al., "Electronic Semiconducting Oxides as pH Sensors", Sensors and Actuators, 5 (1984) 137-146, discuss the limitations of such pH sensors. Oxidizing and reducing agents, such as ferricyanide, ferrocyanide and hydrogen peroxide were found to interfere with pH measurement. Various improvements have been made on the junction-type electrode to make it more rugged and compact.
U.S. Pat. No. 5,110,441 discloses a solid state pH sensor which has an indicator electrode and a reference electrode applied to electrical conductors which are imbedded in an electrically non-conductive substrate. The electrical conductors may be a "cermet material" consisting of ceramic and metallic phases intimately dispersed within one another or metal pins. However, the use of reactive materials, such as the use of brazing compounds, when the conductors are imbedded in the non-conductive substrate, the use of non-inert metallic phases in the cermet material, or the use of reactive metal pins, such as steel pins, can allow corrosive attack caused by the process liquid and by contact between dissimilar materials. Such attack will cause the signal from the pH electrode to drift from the proper measurement and provide incorrect indication of the measured pH.
Thus there exists a need to make a pH electrode which is rugged, compact and free from corrosive attack.